By:
Tim Wusz

“Try
a different racing gasoline? Are you kidding,
they’re all the same.” Don’t bet on it.
You may be missing the easiest and least
expensive performance enhancement there
is by subscribing to the above policy. There
just may be a free lunch.

Many
racers will spend big bucks on new heads,
manifolds, carburetors, etc. before trying
a different liquid in the fuel tank which
could significantly increase performance,
but just pouring in a different gasoline
may not provide an improvement without fine
tuning. It’s just like putting on a new
carburetor without making any adjustments.
If it works perfectly right out of the box,
you probably got lucky.

I
am going to address a few things necessary
to get the most out of your “test” gasoline
so you will not abandon something that may
be a performance improvement.

There
are two main variables that must be considered
when trying a different gasoline. The first
and most obvious is carburetor jetting,
and the second is spark timing. We will
address the jetting issue first.

How
do we know if the carburetor calibration
for the old gasoline is okay for the new
gasoline? If it is not okay, should it be
richer or leaner? To answer these questions,
we need to know the specific gravity (SG)
of the gasoline. Most racing gasoline suppliers
have this information available in their
literature. SG is a measure of how heavy
the gasoline is compared to water. If a
gasoline has a SG of 0.726, this means that
it is 72.6% of the weight of water. The
higher the SG number, the higher the float
sits in the gasoline. This shuts the fuel
flow off earlier at the needle and seat
thereby providing a lower liquid level in
the float bowl. With a low liquid level,
there is not as much pressure from the “head
of gasoline” to help get the fuel moving
through the jets with a given air flow when
compared to a carburetor with a higher liquid
level. The height of the liquid level is
important and should be maintained the same
for each fuel used. That is what the sight
hole in the float bowl is for. Use it.

The
general rule of thumb is that if we are
moving from a higher SG gasoline to a lower
SG gasoline, we need to richen the mixture
by going to larger jets. On the other hand,
if we are moving from a lower SG gasoline
to a higher SG gasoline, we need to lean
the mixture by going to smaller jets.

How
much leaner, or how much richer?? Here is
some ballpark information to get you started.
If the new fuel is lighter (lower SG) than
the old fuel, richen the mixture by one
jet size for every 0.010 difference in SG.
If the new fuel is heavier (higher SG) than
the old fuel, lean the mixture by one jet
size for every 0.010 change in SG. This
will only work if the carburetor was correctly
jetted for the old gas. If we are out to
lunch with the old gasoline, we may still
be out to lunch with the new gasoline.

Example
1:

The
old gasoline has an SG of 0.716, and the
new gasoline has an SG of 0.726. Since the
new gasoline is heavier than the old gasoline,
the mixture needs to be leaner by about
one jet size. If the new gasoline had a
specific gravity of 0.736, the mixture should
be leaned about two jet sizes.

Example
2:

The
old gasoline has an SG of 0.724, and the
new gasoline has an SG of 0.704. The new
gasoline is lighter than the old one, so
the mixture needs to be richer by two jet
sizes since the new gasoline is 0.020 lighter
than the old one.

These
general rules are most accurate with a Holley
four barrel carburetor and racing gasoline.
They may not be accurate with street gasolines
that contain oxygen compounds like MTBE
and ethanol. They also do not apply unless
you adjust the carburetor float level to
provide the correct liquid level.

The
second variable that needs to be addressed
is spark timing. One degree can make a difference,
so don’t get too hasty and move the spark
timing more than one degree at a time. Try
going both ways from where you were optimized
with the previous gasoline to see what combination
works best.

The
reason you should explore the spark advance
arena is because different gasolines can
burn at different rates. This is normally
referred to as flame speed. For maximum
torque, MAXIMUM CYLINDER PRESSURE should
occur at about 12 degrees ATDC. If there
is too much spark timing, the peak pressure
will occur too soon, power will be lost,
and detonation is possible. If the timing
is not enough, then the peak pressure will
occur too late in the power stroke and power
will be lost. Without a fully instrumented
engine and dyno, optimum spark timing (and
maximum horsepower) can be found by watching
lap times in circle track racing, and watching
mph in drag racing.

Let’s
summarize this discussion so we are all
on the same page. If the new gasoline is
heavier (higher SG) than the old one, lean
the mixture by reducing the jet size. If
the new gasoline is lighter than the old
one, richen the mixture. If we don’t have
a clue to the specific gravity of either
fuel, follow this rule that an old guy told
me when I was about 17 years old, “WHEN
IN DOUBT, GO RICHER.” This puts
you in a safe position. If the mixture is
too rich, then lean it down cautiously.

Spark
timing should also be explored due to differences
in gasoline flame speed. If you were “dialed
in” with your previous gasoline, very little
change should be required with your new
gasoline unless it is some special fast
burn blend.